Wednesday, January 14, 2015

Pseudo-science in a pouch: field tests for drugs

Kristen Swann, associate at Morgan Pilate, spoke at the December KC Fed Talk on the topic of unreliable field tests. We asked her to follow up with a blog post, and she graciously accepted our invitation. From Kristen:

John Kelly’s 2008 report False
Positives Equal False Justice
sounds the siren about the nonspecificity of presumptive field tests for drugs,
explaining in lawyer-friendly language how these tests function, exposing the
serious flaws in the existing scientific literature that purports to validate
their use, and documenting the wrongful prosecutions that result from the
tests’ (mis)use. It’s a must-read.

But in the meantime, here is Just Enough
Science to Make You Dangerous. While
I’ll focus here on presumptive tests for marijuana, the same concepts and
limitations apply across the board. Field tests for drugs are
crude investigatory tools.These tests
rely on the principle that certain functional groups – small clusters of atoms
within the larger molecule that determine its chemical properties – will react
with particular reagents to create a particular, ostensibly easily recognizable
color change or series of color changes.But these functional groups are not unique to any particular controlled
substance; they can be found in a number of licit and illicit molecules
alike.The tests look for a structural
feature of the molecule, essentially.

For example, the
Duquenois-Levine (D-L) reagent, for decades the gold-standard of marijuana
field test kits in the United States, does not react specifically with the
complex molecule tetrahydrocannabinol (THC). Rather, it reacts with resorcinol,
a functional group that is present in the THC molecule, but not exclusive to
THC. A substance containing THC will react
with the D-L reagents to change color because of the presence of resorcinol,
but the presence of resorcinol does not indicate the presence of THC – in fact,
resorcinol is a relatively common chemical building block found in Sucrets,
many skin treatments, and a host of innocent plants that likely number in the
thousands. A study cited by Kelly
identified multiple benign, common substances that yield false positives
(cypress, patchouli, eucalyptus) or inconclusive results (lavender, oregano,
spearmint) when tested with the D-L reagent.
In a related Marijuana Policy Project press conference, you can watch
a Hershey’s chocolate bar test positive
for THC. The tests simply are not specific.They cannot positively identify
a substance.

Resorcinol will react with
the reagents in the D-L test to produce, in sequence, “a purple color,” then “a
layering with dark purple on top and light purple on bottom,” or a “dark blue or violet” followed by a
“grey upper layer over a violet layer,” or
a “slate-grey upper layer over a purple lower level,” or “a blue-violet or purple color” that is extracted “into the
lower layer,” depending on how the manufacturer describes a positive result or depicts
it on the packaging. (Compare this with this with this.) Moreover, the color the reagent actually
produces in practice may vary depending upon the concentration of the drug, its
form, or the presence of contaminants, as this oft-cited but Kelly-debunked study that purports to validate these tests freely admits.
If the officer makes the mistake of allowing the test to overdevelop,
they will likely get a false positive, as the chemicals will develop a darker purple
color the longer they are in contact.

Reasonable interior
decorators could disagree about what constitutes slate-grey over purple, what
distinguishes blue from a proper violet, or when a certain hue is really
fuchsia, not purple. How, then, can we
expect police officers to consistently interpret these test results? The
test results are wholly subjective and depend upon the color perception,
training, and inclinations of the officer conducting the test. Further complicating the matter is the
potential for operator error (overdevelopment, for example, or breaking the
ampules in the wrong sequence) and the fact that field tests – conducted, as
they often are, in the field, perhaps in the dark, at the side of a busy road –
are frequently administered in conditions far from ideal for discerning subtle
color changes. And Officer Friendly is
susceptible to confirmation bias: he is
testing the substance because he already believes it to be marijuana, so if
it’s a close call between magenta and purple, he’s likely going to see
purple.

The tests’ limitations are well-known to police officers – but as you hopefully now see, positive test results
may be readily assailed by a defender armed with just a little bit of
science.